An inverter drive system providing easy motor speed control has often been employed for control of a permanent magnet motor used in, for example, a drum type washing machine. Magnetic sensors are provided for detecting respective positions of magnetic poles of a rotor provided with permanent magnets as magnetic poles.
Furthermore, a magnetic flux content (an induced voltage) of the permanent magnets interlinked with a stator coil has been desired to be controlled during a low speed rotation period (a wash step and a rinse step) and during a high speed rotation period (a dehydration step) in the permanent magnet motor used in the drum type washing machine according to a rotational speed of a drum as a load to be driven.
However, the permanent magnets provided in the rotor of the permanent magnet motor are generally composed of a single type of permanent magnet. This results in a normally constant magnetic flux content of the permanent magnets. In this case, for example, when the rotor is composed of only permanent magnets each having a larger coercive force, an induced voltage generated by the permanent magnets rises enormously during a high-speed rotation period (a dehydration step), resulting in a possible insulation breakdown of electronic components or the like. On the other hand, when the rotor is composed of only permanent magnets each having a smaller coercive force, motor output power is reduced during a low-speed rotation period (a wash step or a rinse step).
In view of the above-described problem, a permanent magnet motor has been proposed in which a magnetic flux content of the permanent magnets is adjustable. This permanent magnet motor includes two types of permanent magnets provided in the rotor and having different coercive forces. The permanent magnets each having a lower coercive force are demagnetized or magnetized by an external magnetic field (magnetic field produced by current flowing into the stator coil) so that the magnetic flux content of the permanent magnets is adjusted.
Another proposed permanent magnet motor includes permanent magnets each having a larger coercive force and permanent magnets each having a smaller coercive force, both of which are provided in a part of the rotor interior constituting one magnetic pole. More specifically, one magnetic pole is constituted by a plurality of types of permanent magnets. This enormously increases the number of permanent magnets and necessitates reduction in cubic volumes of the permanent magnets, resulting in a complex structure of the permanent magnet motor.
In order that the above-described problem may be overcome, a permanent magnet motor having a simplified structure is considered in which two types of permanent magnets having different coercive forces are arranged in a rotor at a suitable ratio so that each one magnetic pole is composed of one type of permanent magnets, thereby simplifying the construction.
FIG. 10 shows a permanent magnet motor 100 which is based on the above-described consideration. The permanent magnet motor 100 is an outer rotor type three-phase brushless DC motor and includes a stator 101 and a rotor 102. FIG. 10 is a linear developed arrangement of portions of the stator 101 and the rotor 102 which correspond to two poles. The stator 101 includes a stator core 103 having a number of circumferentially arranged magnetic pole teeth 103a (only one shown) and multiphase or, for example, three-phase stator coils 104 (only one shown) wound on the stator core 103.
The rotor 102 includes a rotor core 105 having a number of circumferentially arranged magnet insertion holes 105a (only two shown) and permanent magnets inserted into the magnet insertion holes 105a respectively. The permanent magnets are divided into two types of permanent magnets 106a and 106b having different coercive forces and are arranged so that each one type constitutes one pole. For example, in FIG. 10, the permanent magnet 106a having a lower coercive force is inserted into one magnet insertion hole 105a (the left one as viewed in FIG. 10) so that a north pole (N) is located at the stator 101 side, for example. The permanent magnet 106b having a higher coercive force is inserted into the other magnet insertion hole 105a (the right one as viewed in FIG. 10) so that a south pole (S) is located at the stator 101 side.
In the above-described case, the rotor core 105 has an inner circumferential surface including portions which correspond to the permanent magnets 106a and 106b and are formed with protrusions 105b (magnetic pole portions) protruding toward the stator 101 side in arc shapes, respectively. A recess 107 is formed between the protrusions 105b adjacent to each other so as to extend to a part of the rotor 102 located between the permanent magnets 106a and 106b adjacent to each other. More specifically, the recess 107 is formed substantially in a middle part of the rotor 102 located between the adjacent permanent magnets 106a and 106b, so as to extend radially with respect to the permanent magnet motor 100. The recess 107 has an inner part located at the outer circumferential side relative to an imaginary central line 108 connecting between a boundary line between the north and south poles of the permanent magnet 106a and a boundary between the north and south poles of the permanent magnet 106b. 
Hall ICs H1, H2 and H3 serving as three-phase magnetic sensors are disposed at one axial end surface side of the rotor 102 at intervals of electrical angle of 120 degrees. The Hall ICs H1 to H3 correspond to a trajectory 109 that is indicated by a line connecting between a line further connecting between pole N and a boundary line (the central line 108) between poles N and S of the permanent magnet 106a and a line connecting between pole S and the boundary line (the central line 108) between poles N and S of the permanent magnet 106b. 
When the rotor 102 is rotated rightward (in the direction of arrow X), the Hall ICs H1 to H3 are moved along the trajectory 109 relatively leftward (in the direction opposed to direction X). The Hall ICs H1 to H3 then generate high-level detection signals corresponding to magnetic pole positions of the rotor 102 respectively. The three-phase stator coils 104 are energized so that the rotor 102 is rotated.
A rotational speed of the drum is low during a wash step or a rinse step of the washing operation, and a high torque and low speed rotation is accordingly required of the permanent magnet motor 100. Accordingly, the permanent magnets 106a are magnetized in order that magnetic flux thereof may be increased. The rotational speed of the drum is high during a dehydration step, and a low torque and high speed rotation is required of the permanent magnet motor 100. Accordingly, the permanent magnet 106a is demagnetized so that the magnetic flux thereof is decreased.
In the above-described construction, a case occurs where the magnetic force Ma of the permanent magnet 106a with the lower coercive force is smaller than the magnetic force Mb of the permanent magnet 106b with the higher coercive force during the dehydration step of the washing operation (Ma<Mb), as shown in FIG. 10. A boundary line Lo of the magnetic forces Ma and Mb is on a line connecting between the central point O located in the inner part of the recess 107 and a central point of the rotor 102 in a normal state where the magnetic forces Ma and Mb of the permanent magnets 106a and 106b are equal to each other. However, a space located nearer the stator 101 side than the inner part of the recess 107 is not a part of the rotor core 105. Accordingly, the space has an enormously large saturating magnetic force. As a result, when the magnetic force Ma of the permanent magnet 106a becomes weaker than the magnetic force Mb of the permanent magnet 106b (Ma<Mb), the boundary line is shown as a boundary line La shifted by angle A (mechanical angle) to the weaker magnetic force Ma side about the central point O. The boundary line La is subjected to a magnetic force Mc from the stator coil 104 thereby to be further shifted in a range of an angle B (mechanical angle) relative to the boundary line La, whereupon the boundary line La is changed into boundary lines Lb and Lc, for example.
In the construction as shown in FIG. 10, the relative movement trajectory 109 of the Hall ICs H1 to H3 passes through the aforesaid space located nearer the stator 101 side than the inner part of the recess 107. As a result, the detection signals generated by the respective Hall ICs H1 to H3 differ from each other. This results in differences among the three-phase current waveforms of the stator coil 104 which are energized based on the respective detection signals thereby to cause torque ripple.